EARLY TERTIARY MAGMATISM IN THE BLACK HILLS, USA

The Black Hills Tertiary igneous suite includes subalkalic, alkalic, and carbonatite magmas. In general, alkalic and carbonatitic magmatism was preceded by subalkalic magmatism, but in detail, the timing of intrusion/extrusion of magmatic products of various compositions was more complex. Earliest magmatism (³58 Ma) was subalkalic trachyte, followed by at least two alkalic silicate pulses (~55-54 and ~49-46 Ma; 40Ar/39Ar). Between the two alkalic silicate pulses were sporadic subalkalic rhyolite eruptions and intrusions (older published K/Ar ages of 56 Ma and 51-52 Ma). A newly published Bear Lodge carbonatite age of 51.3 Ma (Anderson et al., 2013) indicates that carbonatite activity occurred between the last two alkalic silicate pulses and overlapped rhyolite activity. The temporal relationship between rhyolite diatreme activity and carbonatite intrusion is not completely known; however, in smaller centers where it can be determined, rhyolite eruptions preceded alkalic intrusions, such as at the Missouri Buttes west of Devil’s Tower.

Black Hills alkalic rocks include lamprophyre, tephrite, phonolite, alkali rhyolite, and comendite, whereas subalkalic rocks are trachyte and subalkalic rhyolite. Subalkalic rhyolite has the strongest (but variable) crustal signature of any magmatic product in the Black Hills (La/Nb = 1.4; highest initial 87Sr/86Sr; lowest total REE = 105 ppm; La/Yb = 37; Th/U = 2.9, and lowest total Th = 2.6 ppm). REE patterns of subalkalic rhyolites mimic those of Proterozoic greywackes in the Black Hills. These data suggest that earlier crustal-contaminated magmas became more alkalic as less crust was incorporated with time, and magmas traversed better established conduits.

Time-space-composition trends in the Black Hills are similar to those in the Arkansas alkalic province: early lithospheric melts were followed by alkalic magmas, carbonatite and ijolite (e.g. lowest initial ⁸⁷Sr/⁸⁶Sr and highest e_Nd), and final magmatism was more lithospheric. Time-space-composition trends may be related to evolution of magmas and their magmatic conduits, and changes in lithospheric stress. The Black Hills and Arkansas lie along a N40°W trend parallel to the west-coast trench, likely representing upwelling mantle focused by the edges of subducted oceanic plates deep within the mantle.